The invention relates to the removal of salt from salt permeable objects, in particular stone and masonry walls.
The presence and movement of salt in stone and masonry is a major cause of decomposition of stone, brickwork and the like as well as plaster and other masonry coatings. The problem is particularly detrimental to stone walls. It is believed that mineral salts dissolved in water enter a stone wall at the bottom or on one face of the wall. Capillary action pulls the water through or up the wall, the driving force being the evaporation of water at a point remote from the entry point of the water. As the water evaporates, it leaves behind salts, which solidify at or near the surface. The salt crystals which form interfere with the structure of the masonry and lead to surface crumbling. As the outer layer of masonry crumbles away, more salt is drawn to the surface and so the crumbling continues.
The invention has been developed primarily for use in the field of building conservation and restoration and will generally be described hereinafter with reference to that purpose. However, it will be appreciated that the invention is not limited to this particular field of use.
One method of addressing this problem has been to transfer the point of deterioration caused by the salts by applying a poulticing plaster, or sacrificial rendering, to the stone wall to absorb salt from the stone and to provide a sacrificial surface which can be allowed to crumble without undue concern.
Sacrificial renders to date have suffered from a number of drawbacks. Generally, sacrificial renders which are sufficiently absorbent to remove salts and water from stone possess mechanical properties which mean they only last for a few weeks in areas of high salt contamination, disintegrating beyond a useful state in generally too short a time to completely remove salts from the wall. Also, unless stringent maintenance is observed, salts brought to the surface fall to the ground at the base of the wall during the exfoliation process, reverting to a saline solution on contact with ground water that is again drawn through the masonry by capillary action, thereby creating a continuous cycle.
Mechanically stronger renders tend to be less efficient at removing the salt from the wall. Another problem with such renders is that they tend to leave residual material on the stone when they are removed or eventually decompose. Examples of such renders are mixtures of fibreglass fibres with either a sand and lime, sand, cement and lime, or clay base. Attempts at using emulsified pulp alone have led to very unsatisfactory poultices, which shrink upon drying and do not adhere to the wall.
It is an object of the present invention to overcome or ameliorate one or more of the disadvantages of the prior art, or at least to provide a useful alternative.
According to a first aspect, the invention consists in a material for application to a salt permeable object to remove salt from that object, said material including a fluid permeable substrate and an agent for increasing the internal surface area of the material relative to the substrate alone.
In preferred embodiments, the invention is in the form of a poultice (or sacrificial render) with the agent for increasing the internal surface area being diatomaceous earth, and the substrate being cellulose fibres and the invention will be herein described with reference to that particular embodiment, although a skilled addressee would understand that other substrates and agents which increase the internal surface area of the poultice thereby increasing the capillary action of the poultice may also be used.
Preferably, the agent for increasing the internal surface area of the poultice is in a finely particulate form.
Commonly available diatomaceous earth may be used. Diatomaceous earth having particle sizes at the smaller end of the usual range (for example, around 3 xcexcm) provides greater drawing power, however diatomaceous earth of a larger size (for example, up to 1 mm) is also satisfactory in poultices according to the present invention.
Perlite (a rhyolite glass) is another preferred agent which may be used in place of, or in addition to, diatomaceous earth. The agent used must also be inert to the masonry used.
Preferably, the fibrous material contains relatively long fibres, such as would be obtained from the manufacturing or pulping of quality paper, for example fibres in the range 1 mm to 4 mm with a weighted average of 2-3 mm. Without wishing to be bound by theory, it is believed that the individual fibres of the porous fibrous material are coated by diatomaceous earth particles. The two interact, dramatically increasing the capillary suction over either alone. The fibre thereby forms a very effective wick to transfer salt laden fluids away from the masonry, the process being driven by evaporation. Not only do the microscopic and submicroscopic properties of the mixture contribute to the overall function of the present invention, but the mechanical properties are desirable from the point of view of application and quick and clean removal from the masonry surface.
It has been found that a ratio of fibrous material to diatomaceous earth in the range 1:6 to 6:1 has given good results in for most applications. Where the poultice is to be applied to a curved surface, higher ratios of diatomaceous earth may be used to reduce delamination on drying. Where the poultice is to be applied to a flat surface, higher ratios of fibrous material may be used to assist removal from the surface when the poulticing is complete. Poultices with high ratios of fibrous material to diatomaceous earth (up to 6:1) can be subjected to a stronger pull when being removed from the wall and more conveniently come away in larger portions. For most applications, fibrous material to diatomaceous earth in ratios of between 1:2 and 2:1 are preferred, with 1:1 being more preferred.
In highly preferred embodiments, the poultice or sacrificial render contains a binding or thickening agent, such as a cellulose thickener, for example carboxymethyl cellulose.
The poultice or sacrificial render may also contain a biocide. Suitable biocides are those which are commercially available for use with pulps.
Desirably, the poultice or sacrificial render is, when dry, 4-5 mm in thickness.
In a second aspect, the invention also provides a mixture including a material according to the second aspect and water.
This mixture may also include a binder or thickening agent, such as a cellulose thickener, and a biocide. A pH adjusting substance, such as calcium carbonate, may also be added to adjust the pH to desirable levels, usually around pH 7.5.
In a third aspect, the invention provides a method of removing salt from a salt-permeable object including the steps of applying to the salt-permeable object a poultice, said poultice including a mixture of a fluid permeable substrate and an agent for increasing the internal surface area of the material relative to the substrate alone, and retaining said poultice on said surface for a time sufficient for salt to permeate from the salt permeable object into the poultice.
If desired, a fresh poultice may be reapplied after removal of the original poultice.
Preferably, the mixture is applied by means of spray, most preferably from a plastering pump, such as a positive displacement pump at a rate of around 3-5 kg per square meter.
It is preferable that the poultice mixture is applied wet to give a surface coating no more than 20 mm wet thickness and more preferably 8-10 mm wet thickness.
Again, without wishing to be bound by theory, the applicant believes that wet application, as well as providing the necessary adhesion to masonry surfaces activates the salts in the masonry wall and commences the poulticing procedurexe2x80x94in effect, the capillaries in the poulticing mixture are already primed by the presence of water. Thus, when evaporation commences at the surface, the salt laden water within the stone is already in fluid connection with the evaporating water at the surface of the poultice. The water in the mixture thus serves multiple purposesxe2x80x94it provides the material in a slurry form for application to an object, it acts as an adhesive to keep the poultice in place when wet, and it primes the capillaries and dissolves salts in the outer regions of the object, allowing the wicking process to begin.